While an almost endless variety of polymers are known which display a correspondingly broad range of properties, applications are frequently encountered requiring properties not to be found in available polymers. Furthermore, even when polymers with suitable properties are known, they are oftentimes unavailable commercially because of their cost, or due to the fact that the raw materials required to make them are relatively unavailable.
For the preceding and other reasons, therefore, it is frequently desirable to prepare blends from polymers having individual properties that contribute to the polymer combination so as to produce a composition possessing a wide variety of useful physical and chemical characteristics.
While theoretical combinations of polymers displaying required properties are frequently readily conceived, a satisfactory physical joinder of such polymers is sometimes hard to achieve. In this regard, the blending of desirable polymers is commonly attempted by mechanical mixing, or by casting mixtures of polymer solutions. Unfortunately, however, high viscosities of the polymers quite often make their intimate mixing in the form of homogeneous blends difficult.
Furthermore, while the polymeric constituents of some polymer mixtures are relatively compatible in the sense that they show partial miscibility and are relatively easy to combine, others are very immiscible with each other. These latter possess high interfacial tension and often low interfacial adhesion. Such polymers are not only hard to mix because of the magnitude of the internal interfacial tension, but since the transfer of mechanical stress is dependent upon the degree of interfacial adhesion, polymer blends deficient in interfacial adhesion demonstrate poor physical properties, including low tensile strengths, as well as a high order of brittleness. In addition, such blends have a tendency to stratify into gross domains during blending, or during subsequent processing, further impairing their physical properties.
In the past, attempts to circumvent the problems associated with the immiscibility of polymers which it is desired to blend have included the incorporation of copolymers, particularly block copolymers, having separate portions which possess affinities, respectively, for the different polymers making up the blends being prepared. While the technique is often successfully used, it unfortunately entails the expense of providing an additional polymer which usually makes no contribution to the blend other than to preserve its integrity, and in the case of blends fabricated by mechanical means, its efficiency also depends on its own uniform dispersion throughout the blended system.